1. Field of the Invention
The present invention relates to a container having excellent heat resistance, pressure resistance and self-standing ability obtained by biaxially stretch-blow-molding a resin.
2. Description of the Prior Art
Biaxially stretch-blow-molded containers of a thermoplastic polyester such as polyethylene terephthalate (PET) have excellent transparency, surface luster as well as shock resistance, rigidity and gas-barrier property required for the bottles, and have been used as bottles for containing a variety of liquids.
In producing the bottled products, in general, the contents are poured while they are hot or the contents after having been poured are sterilized or pasteurized by heating in order to enhance preservability of the contents. However, the polyester bottles have poor heat resistance and undergo thermal deformation or shrinking and acquire reduced volumes when they are filled with contents that are hot. Because of this reason, the biaxially stretch-blow-molded containers are heat-set after they have been molded.
For those applications (heat-resistant and pressure-resistant bottles) where the bottles are subjected to the sterilization or pasteurization by heating after they have been hermetically filled with the contents which produce pressure by themselves, however, the pressure and heat act simultaneously upon the bottom of the bottles causing them to swell due to heat-creeping phenomenon. The above-mentioned heat-setting only is not quite enough. Therefore, the bottles have been obtained having a round bottom which is provided with a separate skirt member (base cup) (Japanese Laid-Open Utility Model Publication No. 142433/1980 and Japanese Patent Publication No. 30982/1986).
In order to minimize the deformation in the bottom of such a two-piece heat-resistant and pressure-resistant bottle caused by heat and pressure, Japanese Patent Publication No. 22862/1994 discloses technology according to which the center of the bottom which is not drawn or is little drawn is spherulited, a preform of which the center of the bottom portion and mouth-and-neck portion are spherulited, is biaxially stretch-blow-molded in order to draw the whole container, except the spherulited portions, at a high drawing ratio and, particularly, the semi-spherical bottom portion is drawn to reduce the thickness except the center of the bottom portion.
Concerning a method of producing a similar heat-resistant polyester bottle having round bottom, Japanese Laid-Open Patent Publication No. 122516/1988 discloses a method of producing PET bottles wherein a preform is heated at a drawing temperature, the primary blow-molding is effected to produce a primary bottle of a shape larger than a final product, the undrawn or little-drawn thick portions in the bottom of the primary bottle are preheated at a temperature of not lower than 150.degree. C., the whole primary bottle is heat-treated in an atmosphere of a temperature of not lower than 150.degree. C. to increase the crystallinity, the bottle is permitted to freely shrink to form a secondary bottle of a shape smaller than the final product, and the secondary blow-molding is effected in a metal mold having the shape of the final product thereby to obtain a bottle which is the final product.
A polyester bottle of a one-piece structure having pressure resistance, i.e., a petaloid-type bottle, has been proposed already. For example, Japanese Laid-Open Patent Publication No. 154535/1992 discloses a biaxially stretch-blow-molded bottle having a petaloid-type bottom in which a plurality of foot portions are swollen maintaining an equal distance and a valley wall is formed among the foot portions, wherein a central portion including an undrawn peripheral edge surrounding the central flat portion that includes the center of draw of the bottom, is crystallized in a manner that the outer side of the wall of the central portion has a density higher than that of the inner side thereof.
Moreover, Japanese Laid-Open Patent Publication No. 42586/1993 discloses a method of producing a synthetic resin bottle having a self-standing bottom by heat-shrinking the bottom portion only of the secondary article that is primarily blow-molded, followed by the secondary blow-molding.
Japanese Laid-Open Patent Publication No. 85535/1993 discloses a one-piece pressure-resistant self-standing container of which the center of the bottom is spherulited by heating, and further discloses an art wherein a preform of which the center of the bottom is spherulited is subjected to the primary blow-molding to obtain a secondary article having a thick portion that does not extend to the spherulited peripheral edge of the center of the bottom, and the secondary article is subjected to the secondary blow-molding to draw the article except the spherulited portion at the center of the bottom.
The container having a semispherical bottom portion of which the thickness is reduced by drawing exhibits excellent heat resistance and pressure resistance, and sufficiently withstands the heat-sterilization processing (which, legally speaking, is carried out at 65.degree. C. for not less than 10 minutes) of when it is filled with a content such as carbonated beverage and hot water is poured thereon. There, however, exists such inconvenience that a base cup must be prepared separately from the container and must be attached to the container using adhesive or the like.
The self-standing container having a petaloid-type bottom, i.e., having a bottom molded with foot portions as a unitary structure, has an advantage in that there is no need to produce the base cup or to mount it accompanied, however, by a disadvantage in that the heat resistance is not yet satisfactory, i.e., heat resistance and pressure resistance are not yet satisfactory in the bottom portion. That is, in the container of this type, there necessarily exist undrawn or little-drawn thick portions. Under the condition where heat and pressure simultaneously act upon these portions, heat creeping deformation takes place to impair the self-standing ability of the container.
In forming the self-standing container having a bottom portion of the type in which foot portions are molded together as a unitary structure, when a preform of which the center of the bottom and the mouth-and-neck portion are spherulited, is subjected to the biaxial stretch-blow-molding at one time, it becomes difficult to reduce the thickness of the whole bottom portion under highly drawing conditions since the bottom portion has a complex shape. Therefore, little-drawn portions having relatively large thicknesses exist inevitably. The little-drawn portions having relatively large thicknesses exhibit inferior heat resistance or pressure resistance. When such a container is filled with the content and is subjected to the sterilization by heating, it becomes difficult to maintain self-standing ability.
The foot portions that impart self-standing ability to the container have been so formed as to protrude more toward the bottom than the valley portions located on the semispherical surface. Therefore, the foot portions tend to become thin and are often broken during the blow-molding, or lose strength against the pressurized content.
At the valley portions in the petaloid-type bottom and, particularly, at the center of the bottom of the heat-resistant and pressure-resistant bottle, a relatively large force acts thereon locally compared with the barrel portion. Accordingly, the valley portions and, particularly, the central portion in the bottom are deformed in a protruding manner, causing the container to lose self-standing ability and self-standing stability.
Furthermore, according to the method of the above-mentioned Japanese Laid-Open Patent Publication No. 85535/1993, the spherulited peripheral edge of the center of the bottom remains as an undrawn thick portion in the primarily blow-molded secondary article, and the thick portion is drawn by providing split bottom portions from the spherulited peripheral edge of the center of the bottom through the next secondary blow-molding. Even in the container of this type, however, there exists a limitation on the degree of drawing the vicinities of the center of the bottom during the secondary blow-molding. When used for the applications where heat resistance and pressure resistance are required, therefore, the container is deformed due to heat creeping at the time of sterilization by heating. In this case, the spherulited peripheral edge at the center of the bottom of the primarily blow-molded secondary article can be highly drawn to reduce the thickness by providing a spherulited region at the center of the bottom of the preform. It was, however, difficult to obtain a final product having good foot portions by further drawing the highly drawn portions on the bottom of the secondary article through the next secondary blow-molding.
According to the method of the above-mentioned Japanese Laid-Open Patent Publication No. 42586/1993, the thickness at the bottom of the container can be decreased compared with that of a single-stage stretch-blow molding. In the container of an embodiment thereof (FIG. 7, Table 1 of the publication), however, the thickness is 1.6 mm at the center of the bottom, and is still about 1.16 mm in the portions contiguous to the center of the bottom. Therefore, relatively little-drawn portions having a thickness of not smaller than 1 mm tends to remain on the circumference of the center of the bottom. Even the container of this type, therefore, tends to be deformed by heat creeping during the sterilization by heating.